There exist safety devices in motor vehicles which are intended to prevent injuries to persons and animals while automatically closing windows, doors or sunroofs that are operated electrically, pneumatically or hydraulically. In order to detect if a foreign body is clamped between a movable part and a stationary part, it is known to monitor the power consumption, the rotating speed of the driving motor or to utilize temperature sensors which respond and generate a signal indicative of a foreign body situated between the movable part and the stationary part.
German Patent No. 2,926,938 C2 discloses a device for driving a movable element which utilizes the operating current of the driving motor as a value for detecting if a foreign body is clamped between the movable part and the stationary part. In addition, the rotation speed of the motor is mentioned as one additional parameter. A resistor of low value in series with the electric motor is used as the sensor and the voltage, which is proportional to the operating current, is measured across the resistor. Since the motor current increases with the torque generated by the motor, the current increases with the load encountered by the motor. A voltage, which is proportional to the current, is transmitted to a switching logic circuit. If this voltage exceeds a preset value, the electric motor is switched off. In order to make it possible to differentiate between the resistance to movement of the part and instances in which an object is clamped in the opening, both of which cause an increase of the motor current, the increase of the motor current is measured within a predetermined time interval and compared to a cutoff threshold. This cutoff is triggered only if the differential quotient dI/dt exceeds this cutoff threshold and the sum of two successive current values that lie above a first cutoff threshold exceeds a second cutoff threshold. The expenditures associated with the circuit technology for realizing this method are quite high. In addition, it is not ensured that a constant clamping force exists over the entire duration of the closing process. The two cutoff thresholds are preadjusted constants. However, this method is only suitable for isolated instances because the actual values of the cutoff thresholds are subject to fluctuations due to the manufacturing tolerances of the driving motor and the movable elements as well as the mechanical drive elements provided for the movable element. In addition, all components are dependent on temperature and an aging process, both of which alter the values of the cutoff thresholds unpredictably. Consequently, it is not ensured that an evenly maintained permissible clamping force is present as a triggering criterion over the entire duration of the closing process.
German Patent No. 3,303,590 A1 discloses a method in which pertinent data is recorded during the closing process and it is continuously related to a stored power-path diagram of a closing process that was carried out previously in an undisturbed fashion. For this purpose, information about the instantaneous window position, the moving direction of the window and the power consumption of the driving motor is transmitted to a computer unit which subsequently corrects the data of the undisturbed path-time diagram as a function of these parameters, compares said data with a predetermined power tolerance limit and interrupts or reverses the motor current as a function of the result of this comparison. The tolerance limit has different values for the normal range: and the final range of the closing process. This method also utilizes the rotation speed and the power consumption as measuring parameters.